In a large mixing bowl, beat together ¼ cup softened butter, ½ cup sugar, ¼ cup powdered sugar, ½ tsp salt, and ¼ tsp cream of tartar. Mix until combined and fluffy.
Add in 1 egg, 1 tsp baking powder, and slowly add in ½ cup milk, beating together until it forms a thick cake-batter-like mixture.
Pour the batter into the pie crust. It may only come up to halfway fill the pan, but it will expand as it bakes. Reserve ¼ of the cinnamon glaze, and pour ¾ on top of the filling. It may pool or be slightly uneven which is perfectly fine.
Cover the edges of the pie crust with foil and bake at 350°F for 25 minutes.
Remove the foil from the edges of the pan and pour over the remaining glaze. It may sink through the uncooked batter but that's okay. Sprinkle all over with cinnamon sugar.
Bake uncovered for another 20 minutes or until toothpick comes out clean in the center of the pie.
Allow to cool before serving. Serve warm, if possible, and goes wonderfully with ice cream. Or you can chill it and serve cold as a refreshing dessert.